The microzooplankton community during winter in NE Atlantic waters and its potential impact on condition and growth of larval Atlantic herring (Clupea harengus) Dissertation with the aim of achieving a doctoral degree at the Faculty of Mathematics, Informatics and Natural Sciences Department of Biology Institute for Hydrobiology and Fisheries Science at the University of Hamburg submitted by Franziska Bils Hamburg November 2017 Franziska Bils The microzooplankton community during winter in NE Atlantic waters and its potential impact on condition and growth of larval Atlantic herring (Clupea harengus) Dissertation Evaluators of the dissertation: Prof. PhD. Myron A. Peck Dr. Marta Moyano Date of oral defense: 2018/01/26 CONTENTS Contents List of figures ................................................................................................................... IV List of tables .................................................................................................................. VIII Summary .......................................................................................................................... 1 Zusammenfassung ............................................................................................................ 5 1. General Introduction .................................................................................................... 9 1.1 Microzooplankton in the oceanic realm ......................................................................... 9 1.1.1 Unicellular microzooplankton (PZP) and the microbial loop ............................. 10 1.1.2 Metazoan microzooplankton (MZP) .................................................................. 14 1.1.3 Sampling and analyzing microzooplankton ........................................................ 15 1.2 The microzooplankton-ichthyoplankton link ................................................................ 16 1.2.1 Tools to investigate the microzooplankton – ichthyoplankton link ................... 18 1.3 The microzooplankton-ichthyoplankton link in NE Atlantic herring as a case study ... 20 1.3.1 Plankton community in the NE Atlantic ............................................................. 21 1.3.2 Herring in the NE Atlantic ................................................................................... 21 1.3.3 Atlantic herring fisheries and the importance of early life stages ..................... 22 1.3.4 The microzooplankton-larval herring link in the NE Atlantic ............................. 24 1.4 Goals and outline of the thesis...................................................................................... 25 1.5 References ..................................................................................................................... 29 2. Winter protozooplankton community in the North Sea examined using a routine fisheries survey (Manuscript 1) ................................................................................... 37 2.1 Abstract ......................................................................................................................... 38 2.2 Introduction ................................................................................................................... 39 2.3 Material & Methods ...................................................................................................... 40 2.3.1 Sampling ............................................................................................................. 40 2.3.2 PZP biomass, abundance and community composition .................................... 42 2.3.3 Data analysis ....................................................................................................... 43 2.4 Results ........................................................................................................................... 44 2.4.1 Area of study and plankton sampling ................................................................ 44 2.4.2 PZP abundance and carbon biomass ................................................................. 46 I CONTENTS 2.5 Discussion ...................................................................................................................... 51 2.6 Acknowledgements ....................................................................................................... 56 2.7 References ..................................................................................................................... 56 2.8 Supporting information ................................................................................................. 63 3. Exploring the microzooplankton-ichthyoplankton link: A combined field and modeling study of Atlantic herring (Clupea harengus) in the Irish Sea (Manuscript 2) ................. 65 3.1 Abstract ......................................................................................................................... 66 3.2 Introduction ................................................................................................................... 67 3.3 Method .......................................................................................................................... 69 3.3.1 Area of study and plankton sampling ................................................................ 69 3.3.2 Hydrographic conditions .................................................................................... 71 3.3.3 Protozooplankton identification and community composition analysis ........... 71 3.3.4 Larval herring nutritional condition ................................................................... 73 3.3.5 Microzooplankton-ichthyoplankton link ............................................................ 73 3.4 Results ........................................................................................................................... 75 3.4.1 Hydrographic conditions .................................................................................... 75 3.4.2 Micro- and small mesozooplankton abundance and biomass ........................... 76 3.4.3 Protozooplankton community structure............................................................ 80 3.4.4 Herring larvae abundance and nutritional condition ......................................... 82 3.4.5 Microzooplankton-ichthyoplankton link ............................................................ 83 3.5 Discussion ...................................................................................................................... 86 3.5.1 Microzooplankton community composition ...................................................... 86 3.5.2 Spatial distribution of microzooplankton and small mesozooplankton ............ 87 3.5.3 Microzooplankton-herring larvae link ................................................................ 88 3.6 Conclusions .................................................................................................................... 91 3.7 Acknowledgements ....................................................................................................... 92 3.8 Funding .......................................................................................................................... 92 3.9 References ..................................................................................................................... 92 4. The impact of microzooplankton on the nutritional condition and growth of marine fish larvae using stable isotope analysis (δ13C and δ15N) (Manuscript 3) ............................. 99 II CONTENTS 4.1 Abstract ....................................................................................................................... 100 4.2 Introduction ................................................................................................................. 101 4.3 Methods ...................................................................................................................... 103 4.3.1 Plankton and herring larvae sampling.............................................................. 103 4.3.2 Plankton abundance ......................................................................................... 105 4.3.3 Biochemical analysis ......................................................................................... 106 4.3.4 Data analysis ..................................................................................................... 108 4.4 Results ......................................................................................................................... 109 4.4.1 Environmental conditions and larval prey abundances ................................... 109 4.4.2 Herring larvae abundance and distribution ..................................................... 114 4.4.3 Herring larvae trophic position ........................................................................ 115 4.4.4 Herring larvae nutritional condition and growth rate ..................................... 118 4.5 Discussion .................................................................................................................... 120 4.5.1 Larvae spawned later in the year (winter – Downs) rely more on PZP than on MZP to cover their energetic demands ..................................................... 120 4.5.2 Larval herring target different prey items depending on larval size and spawning ground........................................................................................ 123 4.5.3 The survival